The Epidemiology of Diabetic Complications and Interventions (EDIC) study was designed to be implemented immediately upon close-out of the DCCT by the DCCT Research Group, and the final protocol was approved by the participating clinics and the Director of the National Institute of Diabetes, Digestive and Kidney Diseases (NIDDK) in December, 1995. The macrovascular complications of type 1 diabetes include coronary, peripheral vascular, and cerebrovascular diseases. Since the DCCT was not designed to directly assess the development of macrovascular complications, a major question remains as to the risk factors for the acceleration of atherosclerotic vascular diseases in type 1 diabetes. While intensive therapy was associated with an apparent reduction in the risk of cardiovascular disease, there were too few of these events in the DCCT to reliably conclude that improvement of hyperglycemia would ameliorate the risk for macrovascular complications. The morbidity and mortality in type 1 diabetes patients mainly derive from the late-occurring microvascular, neuropathic, and macrovascular complications, usually after 15 to 20 years after the onset of diabetes. Contemporary prospective data on the progression of macrovascular and microvascular complications are limited by 1) failure to separate type 1 diabetes from NIDDM populations; 2) reliance on cross-sectional analyses prone to prevalence bias; 3) inclusion of small, selected populations with limited generalizability; and 4) relatively brief follow-up and significant attrition in prospective studies. Since the DCCT cohort had had an average duration of type 1 diabetes of approximately 12 years by study end, we reasoned that an ideal opportunity exists to study the later manifestations of complications which may occur during a longer follow-up period. Moreover, the DCCT population presented additional advantages for study in that 1) the early course of these complications had been well characterized; 2) reliable, objective outcome measurements were already in place; and 3) established or putative risk factors for complications had been measured and could be repeated in the future. EDIC will therefore examine the DCCT cohort of subjects in a prospective, multicenter, 10-year observational study which would focus on the interactions between established and putative risk factors, including diabetes treatment and the level of glycemic control, for long-term microvascular, neurologic and macrovascular outcomes of type 1 diabetes.